A balun is a transformer between unbalanced electrical transmission lines such as a coaxial cable and microstrip and balanced electrical transmission lines such as parallel twin-lead and twisted pair.
Of the many balun designs described in recent literature, all have an upper frequency limit due to an inherent dependence upon the current flow associated with the guided wave. This is a natural restriction and arises commonly because of the application of principles that work well at lower frequencies such as UHF but become more and more difficult to apply at higher frequencies. Examples of the prior art are the well-known "bazooka" balun designs and that disclosed in U.S. Pat. No. 2,597,853.